Not applicable.
The present invention relates to a method of operating a catalyst bed for the preparation of More particularly, this invention relates to a rotatable catalyst bed comprising a gas to liquid (GTL) catalyst. Still more particularly, the present invention relates to a rotatable bed multiphase catalytic reactor that allows removal of liquid products from the catalyst bed as they form and thus reduces the formation of waxes and other high molecular weight products.
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction entails the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. The reaction is carried out by contacting the hydrogen and carbon monoxide with a catalyst. The reaction gives off a large amount of heat. When the Fischer-Tropsch reaction is carried out in fixed-bed reactors, this high heat of reaction results in an increase in the temperature of the catalyst bed above that of the surrounding environment. Excessive temperature rises can lead to inferior product distribution, and can damage the catalyst if not controlled.
When the Fischer-Tropsch process is carried out in a fixed bed reactor, synthesis gas is fed via an inlet into direct contact with the catalyst while heat is removed from the catalyst bed via heat exchange catalyst and a heat exchange medium, e.g., water. The heat exchange medium is typically contained in one or more tubular conduits passing through the catalyst bed. The optimum temperature gradient between the catalyst and the heat exchange medium must be one wherein the catalyst produces a product having the desired spectrum of hydrocarbons, while the catalyst bed remains thermally stable.
Slurry reactors avoid the problem of paraffinic buildup by suspending the catalyst in the liquid reaction products. On the other hand, the mass transfer rate, which is determined by the ability of the feed gases to reach a catalyst surface, is greatly reduced as compared to fixed-bed reactors. This is because the liquid products of the catalyzed reaction coat the surfaces of the catalyst bed and thus reduce the contact between the feed gases and the catalyst. In addition, catalysis continues while the liquid products remain in contact with a catalyst surface. As the reactions proceed, some of the hydrocarbons grow large enough that they undergo a phase transformation from liquid to solid at reactor conditions. Solids formed in the catalyst bed in this manner are highly undesirable, as they obstruct the surface of the catalyst are relatively difficult to dislodge.
The foregoing issues arise in gas-to-liquid systems, including Fischer-Tropsch systems. Hence, a need exists for a gas-to-liquid system that avoids the inefficiency and operating difficulty caused by the buildup of liquid and solid reaction products in the catalyst bed. It is further desired to provide a system that allows control over the residence time or contact time for the liquid products in the catalyst bed. In addition, the desired system would provide the advantageous mass transfer rate of fixed bed reactors, and yet avoid the need for period removal of solids from the bed.
The present system and apparatus avoids the inefficiency and operating difficulty caused by the buildup of liquid and solid reaction products in the catalyst bed. The present system enhances separation of the products and allows control over the residence time or contact time for the liquid products in the catalyst bed. In addition, the present system provides the advantageous mass transfer rate of fixed bed reactors, and yet avoids the need for periodic removal of solids from the bed. The desired process allows the removal of liquid products from the catalyst bed at a desired rate, which means that the residence time of the liquids in the reactor, and thus the product distribution, can be controlled.
The present invention is applicable to any GTL reactions that are catalyzed by stable solid catalyst, such as Fischer-Tropsch reactions or methanol synthesis reactions.